Ray...and now the simple facts with a couple of tips thrown in as bonus feature. Well actually its another long read for you but, hey information is power.
First the hardware, software at this end if its any help:
A 21.5" Late 2013 3.1 Core i7 self pimped with SSD and 16GB
Running El Capitan 10.1.11
iDVD (v7.1.2) installed just fine from an old iWork 09 disc image. (iMac has no DVD drive)
Photos as it now called cannot be accessed via the Media button. iTunes can?
FYI
UK PAL SD (standard definition) is 720x576px @25fps 4;3 ratio.
iDVD as we know can be set up for that in the preferences.
Please ignore any talk of 640x480 1024x576 etc etc. We are working in PAL, NTSC in fact comes in a number of odd frame sizes.
It is very unlikely that your DVD will be played back on an old 4;3 CRT (tube tv) ...have any of your clients shown a preference for that? Doubt it.
Your iDVD project, PAL, anamorphic, (16:9) widescreen, will be viewed on a HD flat screen tv. @ 1920x1080px. I mention anamorphic because, if by chance it does get viewed on an old 4;3 CRT, then it will be squished horizontally.
Your slideshow images, irrespective of how they came out of the camera..into LR...out of LR ...will be processed by iDVD into a MPEG2 movie @720x576 px.
That DVD video disc will play through a DVD player and be 'upscaled' to 1920x1080px. Yuk
How do we know that?
For the sake of this exercise lets use just 10 images to keep results coming fast.
Also just for fun, bear with me on this, choose 10 landscape images.
I am guessing your original camera files will be 3:2 ratio, most pro DSLRs do. Thats 35mm film.
A pixel size, depending on your pro camera, around the 7360x4912px .
Select all your images in your LR 'ten landscape collection' hit 'D' then 'R' select the 16:9 preset in the Crop and Straighten menu. Then hit Sync, do not click done. Now go through all the images to adjust that crop. I appreciate that this not ideal..but I did say this was just for fun.
Now hit Done. Select all and hit shift/cmd/E .
You now have your LR export window open.
Image Format JPG - Limit File to 2000k - Long Edge 1920 - Resolution 150
Export to your chosen folder.
Open iDVD, create your slideshow project with the images from that folder. Forget the Slideshow scale to TV safe...TV safe only applies to old CRT's which overscan...flat screens do not.
We wont burn at this point, why waste discs, 'Save as VIDEO_TS' ..aha not Disc Image eh!
Hit save and wait...at the appointed destination you find a folder with the name of your project, in that folder will be two folders, see screen shot:

VTS_01_1.VOB will be the mpeg2 video file (_0.VOB is the menu) open _1.VOB with something like QT or mpegstreamclip.
Hit cmd i and you will see the movie info:

...aha, your wonderful camera image is now down to 720x576px @...brace yourself 72dpi. Yep 72 dpi is what video is no matter SD HD or 4K its still only 72dpi. (I know dpi is a print term but for this exercise it will do just fine)
You have a VIDEO_TS folder. (ignore AUDIO_TS its empty)..what can you do with this?
Open DVD Player software and go to File - Open DVD media... select the VIDEO_TS folder and play the DVD in software with no disc present.
Or you can open Toast, select DVD-ROM (UDF) add just VIDEO_TS and burn. It will create a standard DVD video disc just as if you had burnt from iDVD.
Drop that into DVD deck and play on TV...
Want something better? something HD? something to do your pictures justice?
OK you will need the BluRay HD plugin for Toast for this next bit. Its BDMV folders we want.
Find a free HD slide show app such as 4K SlideshowMaker, OK its basic but for this exercise it will do. Stick with the HD for the moment, 4K comes latter,
Drop you LR export images into the app, fettle the prefs, photo duration and switch off Ken Burns then generate a movie, Mac OSX Standard /16:9/High
Open the movie in QT or streamclip hit cmd i and bingo its now a 1920x1080 mp4 (h264) movie.
Fire up Toast. Select the Blu-ray Video button, drop your movie into that.
Again edit the settings mess with the Options...the menus are not that inspiring but hey. turn off auto play and continuous play, encoding if fine at auto for now.
Save as Disc Image this time,
Wait...
Go to the finished disc image, name.toast - double click
a folder opens and you are presented with :

right click on the BDMV file and select Show Package Contents...see that folder STREAM this is where the HD movie is, a transport stream00001, you can open and check with Toast Video Player , QT etc to check the info.

dont move any files in the BDMV..you can copy the mts if you wish .
Come back out of the package view and open Toast select DVD-ROM (UDF) add the BDMV and CERTIFICATE files and burn. It will create a standard DVD video disc just as if you had burnt from iDVD.
??? hang on but this is a BluRay 'project...oh yes you can burn a BluRay structure to a plain old 4.7GB DVD.
As long as you don't go over the limit all the BluRay deck wants to see is the BMDV and CERT files, it does not matter what it is on.
I have clients who use new generation smart tvs and play BluRay content from USB sticks. Even my 5 yr old LG BluRay deck has a USB port which will do the same thing.
You can bypass the whole disc thing and drop the h264 movie file onto a USB if you want...now if you had exported from LR at 3840 long edge you can have a 4K movie, drop on stick and play thru a 4K screen. Now thats a thought.